Adenosine phosphate ester is known to have various physiological functions. To produce a preparation composition that can effectively exhibit the functions of the adenosine phosphate ester, it is necessary to incorporate a certain amount of adenosine phosphate ester in the composition. However, the adenosine phosphate ester is electrolytic, and therefore has a particular electrolyte characteristic, which decreases the strength of the oil/water interface layer in the emulsion composition. Therefore, a common defect in the production of an O/W emulsion composition containing an adenosine phosphate ester is the tendency of coalescence of the emulsified particles, which causes the separation of the water phase from the oil phase, and the emergence of oil or the like from the resulting emulsion. Moreover, even when there is no apparent change, the incorporation of adenosine phosphate ester can be a cause of deterioration of the emulsion system with time, and may rapidly decrease the viscosity of the emulsion.
In view of such problems, there were some attempts to obtain sufficient emulsification stability during the long-term storage of an O/W emulsion composition, for example, by adding a polyglycerin fatty acid ester, acrylic acid-alkyl methacrylate copolymer, alkanoyl lactic acid or its salt during the preparation of the O/W emulsion composition containing adenosine phosphate ester (refer to Patent Literature 1 (PTL 1)). In this composition, dispersion stability is maintained by utilizing electric repulsion of an ionic emulsifier, such as an alkanoyl lactic acid.
In general, a composition containing an ionic emulsifier is stable when the composition has a highly-fluidable liquid form, such as a skin milk; however, when it is a composition having a high viscosity, such as a creamy O/W emulsion composition, the stability of the system cannot be ensured without some kind of additive. Therefore, a method was attempted in which an O/W emulsion system was produced by containing a solid amphiphilic lipid such as a higher fatty acid, a higher alcohol or the like, in addition to the ionic emulsifier. However, when the system contains electrolytes in addition to the above combination of components, the system becomes unstable due to the electrolytes, and the stability of the composition decreases; consequently, it was not possible to maintain the viscosity for a long period. Particularly in the electrolyte-containing composition, separation or other defects of the composition is promoted by changes in temperature. For this reason, it has been difficult to obtain a stable electrolyte-containing composition while ensuring a high viscosity.